The present invention relates to a reflection type projector employing a reflection type light modulating device. The present invention also relates to a reflection type projector that is capable of adjusting a position of a projected image on a screen.
In general, an optical system of a projector includes a light source, a light modulating device and a projection lens.
As the light modulating device, a transmission type device and a reflection type device have been known.
The transmission type projector employs a transmission type modulating device such as a transmission type LCD panel. The microelement of the transmission type LCD panel is a cell that corresponds to a pixel of the projected image.
However, the transmission type projector has a problem in that the light efficiency, which is defined as a ratio of the maximum output light quantity from the modulating device to an incident light quantity onto the modulating device, is deteriorated as the resolution becomes higher in a constant device size. The transmission type modulating device must include opaque portions for arranging driving circuits. In particular, an active matrix type LCD such as a TFT (Thin Film Transistor) has a switching element such as a transistor for each cell. When the resolution becomes higher in a constant device size, the cell size becomes smaller, while the driving circuit can not be reduced as the same ratio as that of the cell. And accordingly, the ratio of the opaque area increases, which deteriorates the light efficiency.
One solution to increase the resolution without deteriorating the light efficiency is to employ the reflection type light modulating device such as a reflection type LCD panel or a DMD.TM. (Digital Micromirror Devices.TM.). Since the driving circuits can be arranged on the backside of the reflection surface in the reflection type light modulating device, it is free from deterioration of the light efficiency due to increase of the resolution.
The DMD.TM. has a large number of micromirrors that are two-dimensionally arranged. The angular position of each micromirror is independently switched between an ON position and an OFF position. The micromirror is a microelement that corresponds to a pixel of the projected image. In the reflection type projector employing the DMD.TM., the light reflected by the micromirrors at the ON position should be separated from the light reflected by the micromirrors at the OFF positions. That is, only the light reflected by the micromirrors at the ON positions should be directed to the projection lens.
Japanese Patent Provisional Publication No. HEI 10-82959 discloses a reflection type projector that employs the DMD.TM.. The disclosed projector is, as schematically shown in FIG. 6, provided with a pair of prisms 1 and 2. Illumination light L1 passes through the first prism 1 and is incident on a first surface 2a of the second prism 2. The illumination light L1 exits from a second surface 2b, and then is reflected by the DMD.TM. 3. Projective light L2 (solid line), which is reflected by the micromirrors at the ON position, is totally reflected by the first surface 2a of the second prism 2 and exits from a third surface 2c to form an image on a screen through a projection lens 4. Non-projective light L3 (dotted line), which is reflected by the micromirrors at the OFF position, exits from a fourth surface 2d of the second prism 2 and is not incident on the projection lens 4.
Since the second prism 2 is designed such that the projective light L2 is reflected by the first surface 2a and the non-projective light L3 passes the fourth surface 2d, these lights L2 and L3 must be spatially separated. Namely, the projective light L2 must not reach the fourth surface 2d and the non-projective light L3 must not reach the first surface 2a. This design requires the long distance from the DMD.TM. 3 to the surfaces 2a and 2d, which increases the size of the second prism 2, disturbing a compact design of the projector. Further, the larger distance from the DMD.TM. 3 to the projection lens 4 requires long back focus with short focal length for the projection lens 4. This restricts freedom of the lens design.
From an another viewpoint, there is a requirement to adjust a position of the projected image on the screen.
Japanese Patent Provisional Publication No. HEI 5-27324 discloses an LCD projector that is capable of adjusting a position of a projected image on a screen. The disclosed LCD projector includes a transmission type LCD panel through which parallel illumination light from a light source is transmitted, a condenser lens and a projection lens. The illumination light is modulated by the LCD panel and projected onto the screen through the condenser lens and the projection lens.
The condenser lens and the projection lens are mounted on an adjusting mechanism so that the lenses are moved, as one unit, in a direction perpendicular to the optical axis. By moving the lenses by the adjusting mechanism, the position of the projection area is shifted on the screen without changing a position of the main body of the projector, and without distortion of the projected image.
The disclosed projector is, however, the transmission type projector and has a disadvantage of the lower light efficiency described above.
In order to separate light paths, in the reflection type light modulating device, an inclined arrangement of the optical elements is required. That is, light paths of an illumination light that is incident on the reflection type light modulating device and a reflected light from the device should be made different from each other. If the adjusting mechanism as disclosed in the publication is employed in the reflection type projector, and the positions of the projection lens and the condenser lens are changed, the reflected light from the reflection type light modulating device is not efficiently incident on the projection lens and accordingly the luminance may be lowered in the projected image and/or part of the projected image may be lost. Accordingly, the adjusting mechanism disclosed in the publication is not applicable to the projector employing the reflection type light modulating device.